An optical device comprising means for detecting a spherical aberration is known from Applicant's U.S. Pat. No. 6,229,600. The invention described in this patent aims at providing a detection system for detecting a spherical aberration. Such a detection system is used in an optical device comprising means for focussing a radiation beam on an information carrier. Information carriers are often scanned through a transparent layer protecting an information layer. A small variation of the thickness of the transparent layer causes a substantial change in the spherical aberration incurred by a high-numerical aperture radiation beam traversing the transparent layer. This spherical aberration might be reduced, by using, for example, a dual lens objective, as described in U.S. Pat. No. 6,229,600. But the amount of spherical aberration might be determined in order to be able to reduce the spherical aberration.
FIG. 1 illustrates how the spherical aberration is detected in an optical device, according to U.S. Pat. No. 6,229,600. The optical device comprises a radiation source 101 controlled by a driver 113, a collimator lens 102, an objective lens 103, a plano-convex lens 104, a beam-splitter 105, two detectors 106 and 107, a signal processor 108, an adder 109, an amplifier 110, and two servo controllers 111 and 112. This optical device is intended for scanning an information carrier 100.
The radiation source 101 produces a radiation beam, which is focussed on an information layer of the information carrier 100 thanks to the collimator lens 102 and the objective lens 103. The radiation beam is reflected by the information layer and is transformed to a converging radiation beam by the objective lens 103 and the collimator lens 102. Part of this converging radiation beam incident on a central zone of the beam splitter 105 is deflected towards detector 106 and part of this converging radiation beam incident on an outer zone of the beam splitter 105 is deflected towards detector 107. The signal detected by detectors 106 and 107 allows the signal processor 108 to provide focus error signals corresponding to the inner part and the outer part of the reflected radiation beam.
The signal processor 108 also determines the spherical aberration of the radiation beam by subtracting these two focus error signals.
Knowing the focus error signals, the servo controller 111 drives an actuator controlling the axial position of the objective lens 103 in order to correct the focus error. Knowing the spherical aberration, the servo controller 112 drives an actuator controlling the axial position of the plano-convex lens 104 in order to correct the spherical aberration.
A disadvantage of the optical device described above lies in the fact that it requires a special beam splitter 105, which makes the optical device bulky and complicated. Another disadvantage lies in the fact that it requires two detectors, which make the optical device bulky and complicated, as these two detectors require a different readout channel, comprising different associated electronics.